Principal Investigator / Author(s): Pitts, James N., Jr.

Contractor: Statewide Air Pollution Research Center, University of California, Riverside

Contract Number: A1-030-32

Using our environmental chambers and spectroscopic and kinetic facilities, we have carried out studies concerning the atmospheric chemistry of long chain n-alkane and aromatic (i.e., benzene and toluene) hydrocarbons, two classes of organics which are major constituents of gasoline and other commercial fuels. In addition, we have concluded our CARB-sponsored investigations of environmental chamber effects. Specifically, we have:

Investigated selected aspects of the atmospheric chemistry of the higher (=C4) alkanes. This study involved the determination of alkyl nitrate yields during the NO, -air photooxidations of the C2-C8 n-alkanes, and the kinetics of the reactions of the n-alkanes and of selected reaction products with the hydroxyl radical. In addition, a series of NO,-air irradiations of n-hexane, n-heptane and n-octane were carried out. The data obtained show that alkyl nitrate formation is an important process for the higher n-alkanes (=C6), especially since the formation of these organic nitrates is a sink for both NO, and radical species. Consistent with these observations, the NO; air photooxidations of the C6-C8 n-alkanes show that these longer-chain alkanes are photochemically unreactive compared to the smaller n-alkanes such as n-butane.

Investigated the atmospheric chemistry of benzene and toluene. We have determined the photolysis rates and the reaction rates with hydroxyl radicals of the a-dicarbonyls glyoxal and methylglyoxal, key photolabile intermediate species formed during the atmospheric photooxidation of these two aromatics. In addition, glyoxal and methylglyoxal yields from benzene and toluene were determined, and NO,-air photooxidations of benzene were carried out. The photolysis rate of the a-dicarbonyl methylglyoxal, which has been postulated to be the major radical source in aromatic photooxidations, was determined to be lower by a factor of 2-7 than previously thought. Since benzene, which cannot form methylglyoxal, was observed to exhibit an unexpectedly high photochemical reactivity, these data show that the present NO, air photooxidation mechanisms of the aromatic hydrocarbons are incorrect in certain aspects and must be reevaluated. This has significant implications for air pollution models currently in use by atmospheric scientists and regulatory agencies, including the CARB.

Concluded our previous studies of chamber-dependent radical sources utilizing the SAPRC 5800-R Teflon-coated evacuable chamber and the 6400-R all-Teflon chamber. The data obtained confirm our previous conclusions that chamber-dependent radical sources are present in environmental chambers. Additionally, we have demonstrated experimentally that nitrous acid is present at part-per-billion concentrations at the commencement of irradiations. While this initial nitrous acid is mainly responsible for the observed radical fluxes in these chambers during the first 30-60 minutes of irradiation, a further chamber-dependent radical source is operative during and after this time period. These results remove one of the major ambiguities in using environmental chamber data to validate reaction mechanisms for photochemical air pollution.

The data from these three research elements are critical inputs into chemical kinetic computer models of the NO, -air photooxidations of fuel constituents and will enhance the utility and reliability of such models in developing emission control strategies.

For questions regarding this research project, including available data and progress status, contact:
Heather Choi
at (916) 322-3893